Rechargeable lithium-ion (Li-ion) and lithium metal batteries (e.g. Li-sulfur, Li metal-air, and lithium-metal oxide batteries) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest capacity (3,861 mAh/g) compared to any other metal. Hence, in general, Li metal batteries have a significantly higher energy density than lithium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode through the electrolyte to the cathode, and the cathode became lithiated. Unfortunately, upon repeated charges/discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately grew to penetrate through the separator, causing internal shorting and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's.
To overcome these safety issues, several alternative approaches were proposed in which either the electrolyte or the anode was modified. The first approach involves replacing Li metal by graphite (a Li insertion material) as the anode. The operation of such a battery involves shuttling Li ions between two Li insertion compounds at the anode and the cathode, respectively; hence, the name “Li-ion battery.” Presumably because of the presence of Li in its ionic rather than metallic state, Li-ion batteries are inherently safer than Li-metal batteries. The second approach entails replacing the liquid electrolyte by a dry polymer electrolyte, leading to the Li solid polymer electrolyte (Li-SPE) batteries. However, Li-SPE has seen very limited applications since it typically requires an operating temperature of up to 80° C. The third approach involves the use of a solid electrolyte that is presumably resistant to dendrite penetration, but the solid electrolyte normally exhibits excessively low lithium-ion conductivity at room temperature. Alternative to this solid electrolyte approach is the use of a rigid solid protective layer between the anode active material layer and the separator layer to stop dendrite penetration, but this typically ceramic material-based layer also has a low ion conductivity and is difficult and expensive to make and to implement in a lithium metal battery. Furthermore, the implementation of such a rigid and brittle layer is incompatible with the current lithium battery manufacturing process and equipment.
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost and performance targets. Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of graphite anode is <372 mAh/g and that of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-200 mAh/g. As a result, the specific energy of commercially available Li-ion cells is typically in the range of 120-220 Wh/kg, most typically 150-180 Wh/kg. These specific energy values are two to three times lower than what would be required for battery-powered electric vehicles to be widely accepted.
With the rapid development of hybrid (HEV), plug-in hybrid electric vehicles (HEV), and all-battery electric vehicles (EV), there is an urgent need for anode and cathode materials that provide a rechargeable battery with a significantly higher specific energy, higher energy density, higher rate capability, long cycle life, and safety. Among various advanced energy storage devices, alkali metal batteries, including Li-air (or Li—O2), Na-air (or Na—O2), Li—S, and Na—S batteries, are especially attractive due to their high specific energies.
The Li—O2 battery is possibly the highest energy density electrochemical cell that can be configured today. The Li—O2 cell has a theoretic energy density of 5.2 kWh/kg when oxygen mass is accounted for. A well configured Li—O2 battery can achieve an energy density of 3,000 Wh/kg, 15-20 times greater than those of Li-ion batteries. However, current Li—O2 batteries still suffer from poor energy efficiency, poor cycle efficiency, and dendrite formation and penetration issues.
One of the most promising energy storage devices is the lithium-sulfur (Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode. The lithium-sulfur cell operates with a redox couple, described by the reaction S8+16Li⇄8Li2S that lies near 2.2 V with respect to Li+/Li0. This electrochemical potential is approximately ⅔ of that exhibited by conventional positive electrodes (e.g. LiMnO4). However, this shortcoming is offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Assuming complete reaction to Li2S, energy densities values can approach 2,500 Wh/kg and 2,800 Wh/l, respectively, based on the combined Li and S weights or volumes. If based on the total cell weight or volume, the energy densities can reach approximately 1,000 Wh/kg and 1,100 Wh/l, respectively. However, the current Li-sulfur cells reported by industry leaders in sulfur cathode technology have a maximum cell specific energy of 250-350 Wh/kg (based on the total cell weight), which is far below what is possible. In summary, despite its great potential, the practical realization of the Li—S battery has been hindered by several obstacles, such as dendrite-induced internal shorting, low active material utilization efficiency, high internal resistance, self-discharge, and rapid capacity fading on cycling. These technical barriers are due to the poor electrical conductivity of elemental sulfur, the high solubility of lithium polysulfides in organic electrolyte (which migrate to the anode side, resulting in the formation of inactivated Li2S in the anode), and Li dendrite formation and penetration. The most serious problem remains to be the dendrite formation and penetration issues.
The traditional Na—S battery holds notable advantages, including high energy density (theoretical value: 760 Wh/kg) and efficiency (approaching 100%), low material cost (rich abundances of Na and S in nature), and long life. All these benefits make them promising for stationary storage applications, for example, utility-based load-leveling and peak-shaving in smart grid, and emergency/uninterruptible power supply. However, this traditional Na—S must operates at a temperature higher than 300° C. The ceramic electrolyte is very brittle and, once a crack is initiated, the system can undergo a catastrophic failure, causing explosion. Thus, a Na—S cell that operates at room temperature is highly desirable.
Sodium metal (Na) has similar chemical characteristics to Li and the sulfur cathode in room temperature sodium-sulfur cells (RT Na—S batteries) faces the same issues observed in Li—S batteries, such as: (i) low active material utilization rate, (ii) poor cycle life, and (iii) low Coulumbic efficiency. Again, these drawbacks arise mainly from insulating nature of S, dissolution of polysulfide intermediates in liquid electrolytes (and related Shuttle effect), large volume change during charge/discharge, and dendrite penetration. Despite great efforts worldwide, dendrite formation and penetration remains the single most critical scientific and technological barrier against widespread implementation of all kinds of high energy density batteries having a Li metal anode.
Most significantly, lithium metal (including pure lithium, lithium alloys of high lithium content with other metal elements, or lithium-containing compounds with a high lithium content; e.g. >80% or preferably >90% by weight Li) still provides the highest anode specific capacity as compared to essentially all other anode active materials. Lithium metal would be an ideal anode material in a lithium-sulfur secondary battery if dendrite related issues could be addressed.
We have discovered a dendrite-resistant, nano graphene-enabled Li metal cell configuration [A. Zhamu, et al., “Reviving Rechargeable Lithium Metal Batteries: Enabling Next-Generation High-Energy or High-Power Cells,” Energy & Environment Science, 2012, 5, 5701-5707]. Each cell consists of a graphene surface-supported Li metal anode and a cathode containing either graphene itself or a graphene-enhanced Li insertion compound (e.g. vanadium oxide) as a cathode active material. Graphene is a single-atom thick layer of sp2 carbon atoms arranged in a honeycomb-like lattice. Graphene can be readily prepared from graphite, activated carbon, graphite fibers, carbon black, and meso-phase carbon beads. Single-layer graphene and its slightly oxidized version (GO) can have a specific surface area (SSA) as high as 2670 m2/g. It is this high surface area that dramatically reduces the effective electrode current density, which in turn significantly reduces the possibility of Li dendrite formation. More specifically, by implementing graphene sheets to increase the anode surface areas, one can significantly reduce the anode current density, thereby dramatically prolonging the dendrite initiation time and decreasing the growth rate of a dendrite, if ever initiated, possibly by a factor of up to 1010 and 105, respectively. However, if a dendrite somehow is formed, this tree-like entity can quickly reach the separator and penetrate through it. It may be noted that this nano-structured graphene layer is used to support a layer of Li foil, which is disposed between the nano-structured graphene layer and the separator layer. There is nothing between the Li metal foil layer and the separator. This graphene nano-structure (behind the Li metal foil) is not capable of stopping or intercepting the dendrite once formed.
Hence, an object of the present invention is to provide a dendrite-stopping layer implemented between the Li or Na metal layer and the porous separator layer in a rechargeable lithium metal battery or sodium metal battery that exhibits an exceptionally high specific energy or high energy density. One particular technical goal of the present invention is to provide a Li metal-sulfur or sodium-sulfur cell with a cell specific energy greater than 500 Wh/Kg, preferably greater than 600 Wh/Kg, and more preferably greater than 800 Wh/Kg (all based on the total cell weight). These must be accompanied by good resistance to dendrite formation, and a long and stable cycle life. Thus, another object of the present invention is to provide a simple, cost-effective, and easy-to-implement approach to preventing potential Li or Na metal dendrite-induced internal short circuit and thermal runaway problems in Li or Na metal-sulfur batteries.